1. Field of Invention
This invention relates to refractory articles such as stoppers that are used as electrical contacts in molten metal casting processes.
2. Background of the Invention
Foundry systems are more complex and technically demanding. The refractories used in them are required to fulfil numerous functions in the metal casting process. One function is to provide a container in which the metal to be cast is melted. Another is to contain the molten metal prior to casting. Still, another is to direct the flow of the molten metal during casting. And, the molds into which the molten metal is poured are also made of refractories. These functions are achieved with a system of refractory vessels, sprouts, channels, nozzles, sliding valves, stoppers or molds. The refractory articles used and their arrangement depends on the design of the molten metal casting system and the corrosiveness of the metal being poured.
Regardless of the system design, the refractories have similar performance requirements. They must be mechanically strong. They must have high temperature softening and melting points. They must resist thermal shock. They must be resistant to the corrosion and erosion of the molten metal. They must not contaminate the metal being poured. They must be economical to make and use. And they must not pollute the environment during use or upon disposal.
Oxides of silicon, zirconium and aluminum have proven to be effective foundry refractories. They are used separately or in combination to form mixed oxide materials such as mullite, fireclay and zircon. These oxides provide low-cost solutions to many foundry casting problems.
Other materials such as graphite and silicon carbide can also be used. These materials are more expensive than their oxide counterparts. They are limited to special applications where the system's requirements justify the added expense. For instance, graphite is used to produce permanent molds. Because of graphite's unique properties of high thermal conductivity and non wetting by molten metals, the use of the more expensive graphite material in molds is justified. Non wetting means that the molten metal does not penetrate or stick to the material's surface. This increases the life of the mold.
A new foundry casting system has been developed that requires that the refractory stopper used to control the flow of the molten metal also conduct electricity. The refractory stopper must provide an electrical link between electrically charged elements of the system. In essence, the refractory stopper must become an electrical switch. This places a new demand on the refractory.
Because refractory oxides, such as silica, alumina, and mullite do not conduct electricity, this proprietary foundry system requires that its stoppers be made from a different refractory material. Since carbon and graphite does conduct electricity, these materials are used to make the stopper. However, the carbon and graphite refractories present the user with new problems.
The first problem is the added expense of using a carbon or graphite. These materials are much more expensive than traditional oxide based refractory materials such as fireclay, kaolin and bauxite. The added cost of the raw material adds to the final cost of the refractory stopper. And unlike permanent molds which are used repeatedly, stoppers are only used once and discarded. Thus, the higher cost is more difficult to justify.
Also, the carbon and graphite materials require more expensive processing techniques. Carbon and graphite oxidize above 950 degrees Fahrenheit. These stoppers are fired at temperatures above 2000 degrees Fahrenheit. They require a controlled atmosphere fire to prevent them from oxidizing. Refractory producers achieve this by placing them in closed containers during firing. Any air leakage into these containers causes the carbon-graphite stoppers to oxidize. This depletes the carbon-graphite and destroys the surface integrity of the stopper.
In addition, most graphite and carbon materials are imported into the United States. Oxide materials such as bauxite and fireclay are readily mined and processed in the United States. This lack of domestic sources could create a shortage of the graphite materials in the U.S. which would seriously curtail the foundry's operation. It is a significant risk factor for foundry planners.
Also, there are fewer refractory producers willing to make carbon and graphite refractory products. Many refractory producers do not have the proper equipment to handle carbon-based materials. Others do not want the added expense of keeping the carbon materials separated from their other product lines. The carbon and graphite materials could contaminate their standard refractory product lines. Fewer producers mean less competition. This creates higher prices. It also could create future shortages if a supplier is shutdown because of labor unrest, natural disasters or financial difficulties.
Finally, the pure carbon or graphite or blended (i.e., carbon-graphite) stopper tends to prematurely solidify the molten metal during the casting operation. This is caused by the very high thermal conductivity of carbon and graphite. This cools the molten metal before the casting operation is completed. This chilling effect creates defects in the final metal casting.
Some of the problems created by the use of a pure carbon-graphite stopper can be reduced by adding clay to the pure carbon-graphite stopper. For instance, the high thermal conductivity of a pure carbon or graphite stopper can be reduced by adding clay to the stopper. This creates a clay-graphite stopper. The clay-graphite stopper has a lower thermal conductivity because the clay has a lower thermal conductivity than either the carbon or the graphite. In effect, the clay addition dilutes the conductivity of the pure graphite stopper.
A clay addition also reduces the cost of the composition because of the much lower cost of clay versus the carbon and graphite raw materials. Depending on the clay used, the cost could be one tenth that of carbon or graphite.
Other problems are also alleviated when clay is added to the carbon-graphite stopper. It strengthens the stopper. This makes it more resistant to mechanical failures and handling defects. It reduces the carbon-graphite content. Thus, the clay-graphite refractory is less dependent on imported raw materials. It also improves the processing characteristics of the material making the stopper easier and cheaper to produce.
However, the clay-graphite composition did not alleviate all the problems of the carbon-graphite composition. For instance, the number of companies willing to produce the clay-graphite stopper is still limited. Whether it is a pure or partial carbon-graphite stopper, the risk of contamination is still present. In addition, the clay-graphite stopper still requires controlled atmosphere firing since it still has a significant graphite content.
In fact, the addition of clay makes the controlled atmosphere firing even more important. Even minor amounts of surface oxidation renders the stopper useless as an electrical contact which is its primary electrical purpose. This occurs because the oxidation of the carbon and graphite at the surface of the clay-graphite stopper exposes only the non conductive clay structure. Thus, the surface of the stopper has the electrical properties of the clay rather than that of the carbon-graphite or clay-graphite surfaces.
Finally, the clay-graphite is a cost improvement, but it is still not as economical as an oxide-based stopper. The amount of carbon and graphite used in a clay-graphite stopper is typically between 35 and 60 percent of the composition, by weight. The cost of the graphite can be 5 to 10 times greater than the cost of the clay component. Thus, the raw materials in the clay-graphite stopper remain significantly more expensive than those of a pure clay or oxide-based stopper. Also, the processing of the stopper remains more costly because of the graphite content.
Thus, the clay-graphite solution still suffers from many of the problems facing the pure carbon-graphite composition.                1) It still contains a high percentage of the more expensive carbon and graphite materials.        2) It still contains a high percentage of the carbon and graphite materials that are, at times, imported and in limited supply.        3) It still requires controlled atmosphere firing which increases its cost of manufacture.        4) It is more prone to surface oxidation during firing which increases product scrap and costs and can render the stopper useless as an electrical contact element.        5) It is still only available from a limited number of refractory suppliers because it still has a significant graphite content and can contaminate their other products.        